The FO switch is a basic building block for many optical applications such as routing in fiber communications networks, photonic signal processing, distributed optical sensing, and optical controls. The desired features for a FO switch include low optical loss (e.g., &lt;1 dB), low interchannel crosstalk (&lt;-30 dB), polarization independence, robustness to catastrophic failure, and simple to align low cost designs for large scale commercial production and deployment. Depending on the application, FO switching speeds can range from nanoseconds to several milliseconds.
Similarly, variable fiber-optic attenuators are the basic building blocks for several key optical systems. Presently, these attenuators are required as equalizers in wavelength division multiplexed (WDM) optical communication systems using non-uniform gain optical amplifiers. Other important applications include polarization dependent loss compensation in fiber optic networks, optical component testing, and optical receiver protection. Hence, a variable fiber-optic attenuator with fast several microseconds duration speed with high attenuation dynamic range (e.g., 35 dB) control is a present challenge to the optical community.
Since centuries, an excellent choice for light control is via the use of mirrors. Mirrors provide high reflectivity over broad bandwidths, as desired in WDM systems. Today, an excellent method for making actively controlled mirrors is via microelectromechanical system (MEMS) technology that promises to offer low cost compact optical modules via the use of low cost batch fabrication techniques similar to semiconductor electronic chip production methods. MEMS technology has been previously proposed to realize fiber optic beam control modules.
For example, in M. F. Dautartas, A. M. Benzoni, Y. C. Chen, G. E. Blonder, B. H. Johnson, C. R. Paola, E. Rice, and Y.-H. Wong, "A silicon-based moving-mirror optical switch," Journal of Lightwave Technology, Vol. 10, No. 8, pp. 1078-1085, August 1992 and N. A. Riza and D. L. Polla, "Microdynamical fiber-optic switch," U.S. Pat. No, 5,208,880, May 4, 1993, FO switches are proposed using the electronically controlled actuation of a single micromirror fabricated using micromaching techniques used in MEMS chip fabrication. More recently, others have used this "single micromirror per optical beam" control approach to realize switches and attenuators. For instance, one such switching module is described in J. E. Ford, J. A. Walker, V. Aksyuk, and D. J. Bishop, "Wavelength selectable add/drop with tilting micromirrors," IEEE LEOS Annual Mtg., IEEE, NJ., postdeadline paperPD2.3, November, 1997, where apart from the limitations of using a single micromirror per beam, this 4-port switch is not reversible and does not form a 2.times.2 switch that can be used to form larger N.times.N switch matrices. Similarly, in S. Glockner, R. Goring, and T. Possner, "Micro-opto-mechanical beam deflectors," Optical Engineering, Vol. 36, No. 5, pp. 1339-1345, May 1997, and L. Y. Lin, E. L. Goldstein, and R. W. Tkach, "Free-space micromachined optical switches with submillisecond switching time for large-scale optical crossconnects," IEEE Photonics Technology Letters, Vol. 10, No. 4, pp. 525-527, April 1998, a single micromirror per beam that can be rather large in size is used, leading to slow millisecond range switching speeds.
Single pixel per beam MEMS-based variable FO attenuators have also been proposed such as described in J. E. Ford and J. A. Walker, "Dynamic spectral power equalization using micro-opto-mechanics," IEEE Photonics Technology Letters, Vol. 10, No. 10, pp. 1440-1442, October, 1998, V. Askyuk, B. Barber, C. R. Giles, R. Ruel, L. Stulz, and D. Bishop, "Low insertion loss packaged and fibre connectorized MEMS reflective optical switch," IEE Electronics Lett., Vol. 34, No. 14, pp. 1413-1414, July 9, 1998, and B. Barber, C. R. Giles, V. Askyuk, R. Ruel, L. Stulz, and D. Bishop, "A fiber connectorized MEMS variable optical attenuator," IEEE Photon. Technol. Lett., Vol. 10, No. 9, pp. 1262-1264, September 1998. Apart from the tolerance limited single pixel control approach, attenuation control in these modules is implemented in an analog fashion by carefully moving a micromirror per beam (or wavelength) through a continuous range of positions. For instance, in both the cited V. Askyuk, et.al. designs, a micromirror is linearly translated to partially block a beam and hence cause attenuation. In the J. Ford, et.al. design case, a micromirror is translated through many small sub-micron size steps to form a varying reflection surface, and this ultra-small motion makes the module very sensitive to vibrations. Thus, extensive module calibration and costly and complex control electronics are required to maintain the high performance of these analog-type FO MEMS-based modules.
To understand the alignment tolerance versus speed dilemma further, for example, in the case of the previously suggested attenuator modules, typically there is one input and one output port. Here, light from a single mode fiber (SMF) for instance is focussed on to a single micromirror that is translated in an analog fashion to act as a variable optical shutter. Although the use of the tiny (a few microns size) optical mirror provides fast response in the microseconds domain, it is also highly susceptible to misalignments with the tightly focussed optical beam. In addition, if the single micromirror fails, the attenuator suffers catastrophic failure and the module completely fails. To improve the alignment problem, researchers have moved to larger micromirrors, although with a drastic reduction in speed to the millisecond regime. Another problem with these previously proposed MEMS-based attenuators is that they are analog devices that require precise analog voltage control, adding to the cost of the component. Hence a design dilemma exists between maximizing speed while maximizing alignment tolerance, simplicity of control, and reduction of component failure probability. The inventions in this patent application solve this dilemma for FO attenuators and switches, particularly using MEMS technology.